Transmission X-ray imaging involves a point source (sometimes referred to as a focus or X-ray focus) of X-rays, a collimator to limit the X-rays to the region of interest and an image receptor to detect the X-rays. When the X-rays pass through an object, X-ray attenuation differences due to structures in the object give rise to differences in transmitted X-ray intensity. These intensity differences are in turn detected by an image receptor giving rise to the detected X-ray image.
The detected X-ray image is composed of two parts. The primary image consists of detected X-rays that have traveled on a straight-line path from the source to the image receptor. The secondary image consists of detected X-rays that have interacted with atoms and electrons in the object and were deflected or scattered from their original path (scattered X-rays). These scattered X-rays fox in a diffuse, out-of-focus image that is superimposed on the primary image. X-ray image contrast is reduced by scattered X-rays with the problem becoming more acute as the thickness and density of the object being imaged increases.
The image receptor, as discussed above, detects the X-ray image. In many cases, the image receptor is a screen/film cassette. The image receptor may also be a computed radiography cassette, a digital flat panel or other devices known in the art. Although the dimensions of such image receptors can vary, the most common cassettes have the following dimensions: 35 cm×43 cm, 30 cm×35 cm, 24 cm×30 cm or 18 cm×24 cm. Common dimensions for digital flat panel image receptors are 43 cm×43 cm and 35 cm×43 cm.
To obtain an X-ray image on the image receptor, the subject is placed between the X-ray focus and the image receptor. For ambulatory or mobile patients, X-ray images are obtained using stationary X-ray systems. In such cases, it is necessary to transport the subject to the stationary apparatus. However, for very sick or otherwise hard to move patients it is often necessary to bring the X-ray apparatus to the subject. In such cases, it is necessary to use a mobile X-ray system. For example, U.S. Pat. No. 6,702,459 to Barnes and Gauntt discloses an improved mobile X-ray imaging system (the contents and disclosure of the '459 patent are hereby fully and completely incorporated herein by this reference). Mobile X-ray systems play an important role in medical imaging, particularly when a subject is ill or cannot be transported easily. Mobile X-ray imaging (also known as portable radiography, mobile radiography and bedside radiography) is accomplished by moving the X-ray unit and image receptor to the subject. Many issues arise in mobile X-ray imaging, including without limitation, difficulties in obtaining optimal alignment of the X-ray beam, increased scatter associated with the images obtained using mobile radiography equipment and problems in positioning subjects to obtain the necessary images.
In tertiary care medical centers, mobile radiographic exams represent a significant percentage of the radiographic exams performed. As discussed above, X-ray scattering is a problem in radiography, particularly mobile radiography. The degrading effect of scatter can be reduced in radiography through the use of anti-scatter grids which decrease the amount of scatter incident on the image receptor. In one embodiment, an anti-scatter grid may include a laminate of lead foil strips interspersed with strips of radiolucent material (FIG. 1). Other types of anti-scatter grids are also known. For example, U.S. Pat. Nos. 6,625,253 and 6,795,529 to Barnes and Gauntt discloses improved anti-scatter grids (the contents and disclosure of the '253 and '529 patents are hereby fully and completely incorporated herein by this reference).
The grid is positioned between the object of interest and the image receptor and oriented such that the image forming primary X-rays are incident only with the edges of the lead foil strips. Thus, the majority of primary X-rays pass through the radiolucent spacer strips. In contrast, scattered X-rays are emitted in all directions after interaction with the object and as such, scattered X-rays are incident on a larger area of lead and only a small percentage of scattered X-rays are transmitted by the grid, as compared to primary X-rays. The degree of scatter control for a given grid depends upon the grid ratio, which is defined as the ratio of the radiopaque strip thickness in the direction of the X-ray path to the width of the radiolucent spacer material as measured orthogonal to the X-ray beam path. Thus, the higher the grid ratio, the greater the scatter control. A high grid ratio, while more effective, is also more difficult to align relative to a focal spot. In order to compensate for X-ray beam divergence a focused grid may be used. In a focused grid, the radiopaque strips are tilted to a greater extent with increasing distance from the center of the grid. The planes of the grid vanes all converge along a line known as the focal axis (illustrated in FIG. 2). The distance from the focal axis to the surface of the grid is referred to as the focal distance of the grid. When the focal spot is coincident with the focal axis of the grid, the primary X-rays have minimal interaction with the radiopaque lead strips and so maximal primary transmission and optimum image quality are obtained. Misalignment of the focal axis of the anti-scatter grid with the focal spot diminishes primary X-ray transmission while scattered X-ray transmission remains unchanged. This reduces the image contrast and thus the image quality. Thus, optimal image quality requires alignment of the focal spot with the focal axis of the anti-scatter grid.
In stationary X-ray systems, the image receptor, anti-scatter grid and X-ray tube are rigidly mounted and in a fixed position relative to one another, thereby making focal spot and grid alignment a simple process. In mobile radiography, however, an image receptor is placed under a bedridden subject and the X-ray source is positioned manually above the subject. Often, to avoid the difficulties in aligning the focal spot with the anti-scatter grid, a grid is not used and thus only a small fraction of the possible contrast is obtained in the X-ray image. As a result, scatter to primary X-ray ratios of 10:1 or more are common in chest and abdominal bedside radiography resulting in less than 10% of the possible image contrast being obtained in mobile radiographic films. Digital image receptors are more sensitive to scattered radiation than screen-film image receptors; therefore, scatter effects are exacerbated when digital image receptors are utilized.
In principle, the contrast in mobile radiography can be improved by using an anti-scatter grid. However, because the image receptor and anti-scatter grid must be positioned manually, it is extremely difficult to achieve proper alignment. When anti-scatter grids are utilized in conjunction with mobile radiography, the anti-scatter grid is typically not well aligned. Misalignment problems are diminished by utilizing an anti-scatter grid having a low ratio of 8:1 or less. With the use of a low ratio anti-scatter grid, X-ray image contrast is often improved compared to using no grid; however, the contrast remains significantly lower than otherwise could be obtained with a properly aligned, high ratio grid having a grid ratio of 10:1 or greater. Thus while mobile radiography is in many ways more convenient than fixed installation radiography its clinical utility is diminished due to the degradation of image quality caused by scattered radiation. Recent developments in aligning the anti-scatter grid with the focal point of the X-ray source in mobile radiography have been developed; one embodiment requires the attachment of a target arm on the side of the image receptor holder, the target arm comprising one or more components of an automatic position measurement system requiring line of sight communications with the mobile radiographic unit (U.S. Pat. No. 6,702,459 to Barnes and Gauntt). In alternate embodiment of U.S. Pat. No. 6,702,459, the image receptor holder comprises one or more components of an automatic position measurement system not requiring line of sight communications.
Most image receptors are rectangular. For some exams, the long axis of the image receptor is oriented parallel to the patient's spine, or craniocaudal axis (head-to-foot axis); this orientation is referred to as “lengthways”. For other exams, the long axis is oriented perpendicular to the craniocaudal axis; this orientation is referred to as “crossways”. The choice of orientation is made based on the exam to be done. For example, chest radiographs are typically done with 35 cm×43 cm cassettes in a crossways orientation, because the 35 cm dimension is long enough to see the full height of the region of interest, while it is very important to image the full width of the patient. In contrast, abdominal radiographs are typically done with the cassette in a lengthways orientation, because it is more important to image the full length of the abdomen and less important to obtain images of the periphery of the patient.
The greatest improvement in image contrast in the spine is achieved when the focal axis of the anti-scatter grid is approximately parallel with the craniocaudal axis (head-to-foot axis). A conventional grid tunnel may have the focal axis parallel to the craniocaudal axis for crossways radiographs, but will then have the focal axis perpendicular to the craniocaudal axis for lengthways radiographs. Thus, a single conventional grid tunnel cannot take optimal radiographs in both the lengthways and crossways orientations, no matter how accurately the focal spot is aligned with the focal axis.
Digital flat panel detectors are becoming increasingly commonly used as image receptors in bedside radiography. These detectors can transmit an image directly to a computer without being removed from under the patient. These detectors are sometimes equipped with low ratio, removable anti-scatter grids; such grids are known in the trade as grid caps. Some manufacturers provide square flat panel detectors. The transverse and longitudinal dimensions of a square image receptor are identical, so the terms “crossways” and “lengthways” are not relevant for these detectors. Such a detector equipped with a high ratio grid cap would provide optimal and flexible imaging for bedside radiography provided that the focal spot can be aligned with the grid focal axis. Barnes and Gauntt disclose a system to provide such alignment to a grid tunnel with a removable image receptor, but they do not explicitly describe alignment to digital flat panel image receptor with a removable grid cap. An additional concern is ease of use. Placing a 35 cm×43 cm cassette under a 300 pound unconscious patient is not an easy task. Roddy (U.S. Design Pat. D397,795) disclosed a cassette holder to simplify the tasks of placing the image receptor under the patient, and of removing the image receptor from under the patient. This is accomplished by placing handles along the short side of the cassette holder. When the cassette is oriented for a crossways radiograph, the handles are along the patient's side and thus easily accessible. However, when oriented for lengthways radiography, the handles are under the patient and do not aid in removing the image receptor. McNair (U.S. Design Pat. D546,453) discloses a grid tunnel with handles along one long and both short sides of the grid tunnel. This approach aids in handling for both lengthways and crossways orientation, but is bulkier than Roddy's design. Furthermore, it is impossible with McNair's design to acquire both crossways and lengthways radiographs with the grid focal axis parallel to the patient's craniocaudal axis.
A further concern is the thickness and stiffness of the cover of the image receptor assembly. Grid tunnels, film-screen and computed radiography cassettes, flat-panel digital image receptors, and similar image receptor assemblies, are all equipped with a cover that protects the interior of the assembly from the environment. For example, the interiors of film/screen cassettes and photodiode flat-panel digital image receptors needs to be protected from light, and all image receptor assemblies need to be protected from dust and mechanical injury. To provide maximum protection against mechanical injury it is preferable to use a thick cover; however, to minimize X-ray absorption and scattering (collectively, X-ray attenuation) it is preferable to use a thin cover.
Accordingly, there exists a need for a bi-directional image receptor assembly useful in radiography, particularly mobile radiography, that can accept rectangular image receptors in either a lengthways or crossways orientation while maintaining the proper anti-scatter grid alignment in respect to the subject's craniocaudal axis. There also is a need for a digital flat panel detector and associated grid cap that is compatible with the alignment system disclosed by Barnes and Gauntt, and a need for a protective cover that simultaneous provides low X-ray attenuation and high mechanical stiffness.